As a conventional diversity transmitting method, there has been investigated a method of using a space-time coding (STC) for transmitting a transmission signal of the same content by arranging plural antennas in a radio transmitting apparatus, and by shifting time from each antenna. According to STC, the radio transmitting apparatus can obtain transmission diversity gain even when the apparatus does not know channel information (channel quality) between the subject transmitting apparatus and a radio receiving apparatus.
As an example of STC, there is STBC (Space Time Block Coding) (for example, see non-patent literature 1, non-patent literature 2, and non-patent literature 3). For example, a case where two antennas (antennas #1 and #2) are used in STBC in the radio transmitting apparatus and one antenna is used for the radio receiving apparatus is explained. The radio transmitting apparatus transmits data signals s(m) and s(m+1) at symbol times in and (m+1) of continuous times (for example, time mT and time (m+1)T).
In this case, the radio transmitting apparatus performs space-time coding to the data signals s(m) and s(m+1) at the two symbol times m and (m+1) (time mT and time (m+1)T), and transmits space-time coded data x1 that is transmitted from antenna #1 and space-time coded data x2 that is transmitted from antenna 42, to the radio receiving apparatus, as expressed by following equation 1. However, the upper subscript “*” shown in equation 1 denotes a complex conjugation.x1(m)=s(m)x2(m)=s(m+1)x1(m+1)=−s*(m+1)x2(m+1)=s*(m)  Equation 1
Meanwhile, the radio receiving apparatus performs space time decoding by using reception signals yi(m) and yi(m+1) that are received at the two continuous symbol times m and (m+1) (at time mT and time (m+1)T). In following equation 2, hi1 denotes a channel estimation value between antenna #1 of the radio transmitting apparatus and the antenna of the radio receiving apparatus, and hit denotes a channel estimation value between antenna #2 of the radio transmitting apparatus and the antenna of the radio receiving apparatus. Further, zi denotes a noise signal, and i denotes a port number of a receiving antenna (in this case, a number of receiving antennas is 1, and therefore i=1).
                    Equation        ⁢                                  ⁢        2                                                                                                                                  yi                  ⁡                                      (                    m                    )                                                  =                                ⁢                                                      hi                    ⁢                                                                                  ⁢                    1                    ⁢                    x                    ⁢                                                                                  ⁢                    1                    ⁢                                          (                      m                      )                                                        +                                      hi                    ⁢                                                                                  ⁢                    2                    ⁢                    x                    ⁢                                                                                  ⁢                    2                    ⁢                                          (                      m                      )                                                        +                                      zi                    ⁡                                          (                      m                      )                                                                                                                                              =                                ⁢                                                      hi                    ⁢                                                                                  ⁢                    1                    ⁢                                          s                      ⁡                                              (                        m                        )                                                                              +                                      hi                    ⁢                                                                                  ⁢                    2                    ⁢                                          s                      ⁡                                              (                                                  m                          +                          1                                                )                                                                              +                                      zi                    ⁡                                          (                      m                      )                                                                                                          ⁢                                  ⁢                                                                              yi                  ⁡                                      (                                          m                      +                      1                                        )                                                  =                                ⁢                                                      hi                    ⁢                                                                                  ⁢                    1                    ⁢                    x                    ⁢                                                                                  ⁢                    1                    ⁢                                          (                                              m                        +                        1                                            )                                                        +                                      hi                    ⁢                                                                                  ⁢                    2                    ⁢                    x                    ⁢                                                                                  ⁢                    2                    ⁢                                          (                                              m                        +                        1                                            )                                                        +                                      zi                    ⁡                                          (                                              m                        +                        1                                            )                                                                                                                                              =                                ⁢                                                                            -                      hi                                        ⁢                                                                                  ⁢                    1                    ⁢                                                                  s                        *                                            ⁡                                              (                                                  m                          +                          1                                                )                                                                              +                                      hi                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                                                                  s                        *                                            ⁡                                              (                        m                        )                                                                              +                                      zi                    ⁡                                          (                                              m                        +                        1                                            )                                                                                                                              [        2        ]            
The radio receiving apparatus separately detects data signals s(m) and s(m+1) by using the equation 2, as expressed by following equation 3.[3]hi1*yi(m)+hi2yi*(m+1)=(|hi1|2+|hi2|2)s(m)+hi1*zi(m)+hi2zi*(m+1)hi2*yi(m)−hi1yi*(m+1)=(|hi1|2+|hi2|2)s(m+1)+hi2*zi(m)+hi1zi*(m+1)  Equation 3
From the equation 3, a reception SNR (Signal to Noise Ratio) is expressed by following equation 4, and it is clear from the equation that maximum ratio combining diversity effect of two branches is obtained. In following equation 4, Ps denotes signal power, and Pz denotes noise power. In this way, in STBC, transmission diversity gain can be obtained even when the radio transmitting apparatus does not know channel information.[4]SNR=(|hi1|2+|hi2|2)Ps/Pz  Equation 4
In the following explanation, a number of antennas that are used in STBC by the radio transmitting apparatus is called “the number of dimensions in space diversity”. A time during which space-time coded data signals are continuously transmitted is called a “transmission block interval”. That is, in the above-described example of STBC, because the number of antennas of the radio transmitting apparatus is two, the number of dimensions in space diversity is 2. Because the time during which data signals are continuously transmitted is two symbol times m and (m+1), a transmission block interval is 2.
It is anticipated that a preamble sequence to estimate channel quality between the antenna of the radio transmitting apparatus and the antenna of the radio receiving apparatus that becomes necessary when decoding a space-time coded signal by using STBC is transmitted by being superimposed with data signals. In this case, because preamble sequences are preferably simultaneously transmitted at each dimension of space diversity (antenna), it is necessary to allocate mutually different preamble sequences for each antenna used in STBC. The radio receiving apparatus detects a preamble sequence from each antenna of the radio transmitting apparatus, and estimates a channel between each antenna of the radio transmitting apparatus and the antenna of the radio receiving apparatus.
It has been investigated that the radio receiving apparatus blind detects a preamble sequence of each antenna (see Patent Literature 1, for example). Specifically, first, the radio transmitting apparatus and the radio receiving apparatus share candidate data of a preamble sequence to be used. The radio transmitting apparatus fixedly allocates a preamble sequence to each antenna of the radio transmitting apparatus. The radio transmitting apparatus uses an antenna to which a preamble sequence of a sequence number n corresponding to the number of dimensions (n, for example) of space diversity is allocated, as a main antenna, and uses antennas other than the main antenna as subordinate antennas. In this case, a preamble sequence of the sequence number n corresponding to the number of dimensions n of space diversity is called a main antenna preamble sequence.
For example, when the number of dimensions n of space diversity is 4, the radio transmitting apparatus transmits a preamble sequence by using four antennas (antennas #1 to #4, for example). In this case, the radio transmitting apparatus fixedly allocates preamble sequences of sequence numbers #1 to #4 to antennas #1 to #4, respectively. That is, the preamble sequence of the maximum sequence number #4 corresponding to the number of dimensions n=4 of space diversity becomes the main antenna preamble sequence. Antenna #4 to which the preamble sequence of sequence number #4 (the main antenna preamble sequence) is allocated becomes the main antenna, and antennas #1 to #3 become the subordinate antennas.
On the other hand, the radio receiving apparatus blind detects preamble sequences, by correlating between candidate data of the preamble sequence shared with the radio transmitting apparatus and the received preamble sequence. The radio receiving apparatus specifies a preamble sequence of a maximum sequence number as the main antenna preamble sequence among blind detected preamble sequences, and sets the sequence number of the specified preamble sequence as the number of dimensions in space diversity. For example, when the radio receiving apparatus blind detects preamble sequences, and also when the preamble sequence of a maximum sequence number among the detected preamble sequences is the preamble sequence of sequence number #4, the radio receiving apparatus specifies the number of dimensions in space diversity as n=4. The radio receiving apparatus estimates channel quality by using the detected preamble sequence, and performs space-time coding, based on the obtained channel estimation value and the number of dimensions n of space diversity.